All about Intellectual Disability (Mental Retardation)


What is disability?

A disability refers to personal limitations that are of substantial disadvantage to the individual when attempting to function in society. A disability should be considered within the context of the individual’s environmental and personal factors, and the need for individualized supports. Intellectual disability is associated with low level of intelligence

What is intelligence?

Intellectual Disability

Intelligence refers to a general mental capability.

It involves the ability to:

  • Reason
  •  Plan
  •  Solve problems
  •  Think abstractly
  •  Comprehend complex ideas
  •  Learn quickly
  •  Learn from experience.

What is intellectual disability?

Intellectual disability, formerly known as mental retardation, is a disability characterized by significant limitations both in intellectual functioning (reasoning, learning, problem solving) and in adaptive behavior, which covers a range of everyday social and practical skills. This disability originates before the age of 18.

Intellectual Disability

DSM 5 Diagnostic Criteria for Intellectual Disability:

Intellectual disability (intellectual developmental disorder) is a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains. The following three criteria must be met:

  1. Deficits in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by both clinical assessment and individualized, standardized intelligence testing.
  2. Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, across multiple environments, such as home, school, work, and community.
  3. Onset of intellectual and adaptive deficits during the developmental period.

What is Adaptive Behavior?

Adaptive behavior represents the conceptual, social, and practical skills that people have learned to be able to function properly in their everyday lives. Significant limitations in adaptive behavior have a great impact a person’s daily life and affect the ability to respond to a particular situation or to the environment.  The person with disability may have problem in social, conceptual, and practical domain.

Intellectual Disability

Conceptual skills:

Conceptual skills include the use and understanding of language, reading and writing, and problem solving etc.

Social skills:

Intellectual Disability

Social skills are very important for the person because these are the social skills which guide the person to obey the law, follow the rules, make interpersonal relationships, avoid victimization, and the most important is gullibility. It also helps the person to develop self-esteem.

Practical skills:

Practical skills focus on the daily life activities such as eating, making meals, taking medicine, mobility, toileting, doing housekeeping activities, using the cell phone etc.

Levels of ID

There are four levels of intellectual disability based upon the IQ level of the person.

Mild intellectual disability

IQ 50 to 70 (85 percent of people with intellectual disabilities).

Moderate intellectual disability

IQ 35 to 49 (10 percent)

Severe intellectual disability

IQ 20 to 34 (3 or 4 percent)

Profound intellectual disability

IQ less than 20 (1 to 2 percent)

Prevalence of ID

Intellectual Disability

According to the World Health Organization (WHO) of 2001, almost 3% of the world population has some form of ID. According to the reports between 2 to 6 people in 1,000 is suffering from ID. The ratio of the disability is greater in male population as compared to the females with the ratio of 1.6:1 (Mild) and 1.2:1 (Severe).

Etiology of ID

Etiology of intellectual disability means its causes. There are many different causes of ID but about 30 to 50 percent of the causes are still unknown. Among the known causes there are many factors including genetic, environmental, prenatal, and perinatal factors etc. About 3% of the whole world population have an IQ less than 70. Down syndrome is also one of the most important causes of Intellectual disability affecting 1 among 800-1000 births.

Intellectual Disability

Genetic Factors:

Many of these are associated with atypical or dysmorphic physical characteristics (Sultana et al., 1995). Such conditions include fragile X syndrome, neurofibromatosis, tuberous sclerosis, Noonan’s syndrome and Cornelia de Lange’s syndrome. The children with some chromosomal abnormalities like Down syndrome may be recognized due to some dysmorphic physical characteristics of the disease but in the problems like Klinefelter’s syndrome (47,XXY), no surch characteristics are obvious to the physician or the family members.

Prenatal problems:

Prenatal problems are the problems which may cause the problems like intellectual disability before birth. These causes include congenital infections such as cytomegalovirus, toxoplasmosis, herpes, syphilis, rubella and human immunodeficiency virus; prolonged maternal fever in the first trimester; exposure to anticonvulsants or alcohol; and untreated maternal phenylketonuria (PKU). Moreover complications in pregnancy especially the low birth weight babies are more prone to have intellectual disability. Beside these causes some diseases in mother such us heart and kidney disease and diabetes, severe prematurity, birth asphyxia, difficult and/or complicated delivery and birth trauma are also important causes of intellectual disability.


Infancy and Childhood Problems  

The four weeks following birth (neonatal) are very crucial for the health of the infant. Head injury, exposure to lead, brain infections such as tuberculosis, Japanese encephalitis, and bacterial meningitis, and malnutrition may also be the causes of intellectual disability.

Iodine deficiency

Iodine deficiency is among the major causes of intellectual disability. Iodine deficiency is currently affecting about 2 billion people in the world. If the mother do not consume the required amount of iodine, it can affect the foetus, this condition is called hypothyroidism. Beside the intellectual disability iodine deficiency is also the major cause of goiter, and enlargement of the thyroid gland.

Metabolic disorders

Metabolic disorders can also cause intellectual disability. The disorders like PKU, hypothyroidism can be cured by the early interventions while mucopolysaccharidosis and sphingolipidoses are less responsive to early intervention

Disease or toxins

There are some diseases which may be the causes of intellectual disability if not properly treated these include whooping cough, measles, or meningitis. Exposure to lead and some other chemicals is also a big cause of ID.


There are some countries like Ethiopia and some other African countries where there the food is less than the demand of people, and also due to some natural disasters like flood, earthquake, famine etc may cause malnutrition which is also a major cause of intellectual disability.


American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th Edition. Washington, DC: APA Press; 2013

Arvio, M., & Sillanpaa, M. (2003). Prevalence, aetiology and comorbidity of severe and profound intellectual disability in Finland. Journal of Intellectual Disability Research, 47, 108–112.

Daily, D.K., Ardinger, H.H., & Holmes, G.E. (2000). Identification and evaluation of mental retardation. American Family Physician, 61(4):1059-1067

Durkin, M. (2002). The epidemiology of developmental disabilities in lowincome countries. Mental Retardation and Developmental Disability Research Reviews, 8, 206–211.

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